1. Field of the Invention
The present invention relates to a relief valve structure for an oil pump of an engine of a vehicle, the relief valve being capable of appropriately controlling/switching the discharge pressure and flow rate of oil to an optimum condition in low, middle and high speed regions of the engine.
2. Description of the Related Art
Lubricant oil is supplied to an engine of a vehicle through an oil pump by carrying out engine operation. Most oil pumps are installed with a relief valve for controlling the pressure of the oil. The relief valve device evens out the pressure of the oil by releasing the oil to a different route when the pressure of the oil rises in the oil pump, in order to prevent harmful effects to other devices.
However, relief operation of the relief valve drops the pressure of the oil from high to low without stopping, and therefore the amount of oil supply pulsates without being constant, generating bubbles in the oil. As a result, the relief valve devices tarts vibrating, causing possible harmful effects to the other devices to be supplied with the oil. Therefore, some relief valve devices are designed to return the oil from the relief valve a little bit at a time in the initial stage, in order to prevent rapid drop of the pressure.
Furthermore, recent oil pumps are required to perform more meticulous control. For example, the discharge pressure and flow rate of oil are reduced to realize high efficiency in a certain rotation speed region, but a volume of discharge pressure and flow rate of oil are ensured in order to secure lubrication in another rotation speed region. The requirement of such characteristics is attributed to the following fact. In other words, in the cold months of the year, the engine oil cools down when the engine is not activated.
As a result, the viscosity of the oil increases, and consequently the discharge pressure of the oil pump increases when the engine is started in this state. Then, the discharge pressure of the oil pump reaches the highest level when the engine is rotated to the maximum speed with the cold engine oil. Overly high discharge pressure in such circumstances causes a strain on the oil filter and pipe system, and the amount of unnecessary work increases, causing harmful effects to the abovementioned other devices.
However, in a rotation speed region where the discharge pressure exceeds a predetermined value once and the relief valve is already opened, a conventional relief structure generally cannot control the discharge pressure by further opening the valve. Note that an air vent hole is formed on the deeper side of the relief valve so that the relief valve can smoothly move in the axial direction. The capacity on the deeper side of the relief valve changes significantly as the relief valve moves in the axial direction.
By allowing the air to be drawn/discharged to/from the air vent hole, the relief valve can smoothly move in the axial direction. In other words, the space on the deeper side of the relief valve is in the form of a so-called closed chamber, which is configured such that the volume of the space on the deeper side of the relief valve cannot fluctuate without a hole through which air can pass to the outside. Because the outside of the air vent hole is in the form of an oil pan, there is little difference whether the substance to be drawn/discharge is air or oil.
The prior art documents, Japanese Utility Model Registration No. 2543058 and Japanese Patent Application Publication No. H5-195742, disclose how the discharge pressure is controlled in the relief valve single mechanism (achieving high efficiency by reducing the amount of unnecessary work, as well as a balance between lubricity and reliability by ensuring discharge pressure). Generally, when performing a control for increasing and reducing the discharge pressure in multiple stages, in most cases a plurality of opening parts are provided on a side surface of a valve passage. The relief valves moves in the axial direction as the discharge pressure increases or decreases, whereby the number openings on the side surface of the valve passage increases or decreases, and the area of opening increases to relieve the oil. Consequently, the amount of oil relieved and the discharge pressure can be increased or decreased.
As already described, in an oil pump the discharge pressure and the discharge flow rate are substantially proportional to the rotation speed of the pump. However, from the perspective of the engine system, broadly speaking the required oil pressure and the required oil flow rate are apt to increase logarithmically in relation to the rotation speed (the more the rotation speed increases, the more the increasing rate of the required oil is reduced). In other words, when comparing the discharge pressure of the pump with the pressure required by the engine system on the basis of the rotation speed, the higher the rotation speed is, the greater the deviation of the pump supply pressure and the pressure required by the engine system.
From the phenomenon described above, clearly, it is preferred to carry out a control for gradually increasing the area of opening for relieving the oil (the amount and pressure of oil to be relieved) as the rotation speed increases. In the actual operation, as the discharge pressure rises, the spring shrinks gradually, whereby the relief valve recedes. As a result, the number of through-holes for opening the relief opening parts provided on the side surface of the valve passage increases. Unnecessary amount of work can be reduced by performing the control for further increasing the oil pressure to be relieved (but the discharge pressure is not increased significantly) when the rotation speed increases as described above.
In so doing, the area of opening of each relief opening parts provided on the side surface of the valve passage needs to be larger on the deeper side of the valve. When the area of opening of the relief opening parts is larger on the deeper side of the valve, the oil pressure to be relieved can be further increased as the rotation speed increases. In other words, for example, when the relief opening parts having a larger opening area are arranged on the near side of the valve passage to prevent the increase of the discharge pressure, the oil is relieved first from the relief opening parts with a larger opening area in a middle speed region where the oil does not need to be relieved. Consequently, a large amount of oil is relieved and the discharge pressure is reduced, whereby the lubricity becomes inadequate.
When the oil pump requires a certain level of oil pressure to ensure lubricity and reliability in a certain rotation speed region only, the relief valve that is opened in the middle speed region once needs to be closed again in certain middle to high speed regions higher than the middle speed region, to ensure the oil pressure. In the middle to high speed regions higher than the middle speed region, the relief valve is receded to the further deeper side by high discharge pressure. Therefore, the first relief path 3 of Japanese Utility Model Registration No. 2543058 and the first relief hole 3a of Japanese Patent Application Publication No. H5-195742 that are in “front” of the relief valve opening in the middle speed region cannot be left closed.
Thus, in the piston-shaped valve body 7 of Japanese Utility Model Registration No. 2543058 and the sleeve 7 of Japanese Patent Application Publication No. H5-195742 are disposed to close the first relief path 3 and first relief hole 3a in front of the relief valve at the middle to high speed regions. In other words, the piston-shaped valve body 7 and the sleeve 7 are required in Japanese Utility Model Registration No. 2543058 and Japanese Patent Application Publication No. H5-195742 respectively, as the special members for closing, again, the first relief path 3 of Japanese Utility Model Registration No. 2543058 and the first relief hole 3a of Japanese Patent Application Publication No. H5-195742 that are opened once, in the region of higher rotation speed.
Furthermore, the relief valve is provided with a hole for closing the opened relief opening parts in the middle to high speed regions. For example, this hole is the first valve hole 7B in Japanese Utility Model Registration No. 2543058 and the sleep hole 7a in Japanese Patent Application Publication No. H5-195742. The discharge pressure is controlled by opening and closing these holes with respect to the relief opening parts. In the low speed region where idling is also performed, because the rotation speed is low and therefore the discharge pressure, the oil is not relieved from the relief valve, and all of the relief opening parts are closed.
Thereafter, the relief valve recedes to the deeper side and opens or closes as the discharge pressure increases, the hole provided in the relief valve is located in a position further ahead of the relief opening part that is in the very front part of the low speed region in both Japanese Utility Model Registration No. 2543058 and Japanese Patent Application Publication No. H5-195742. This configuration causes the following problems. Generally, the oil is relieved from the relief opening parts, but when the through-hole provided in the relief valve is positioned in front of the relief opening parts in the axial direction, the oil oozing out of the through-hole of the relief valve cannot reach a sliding surface between the relief valve and the valve passage. As a result, the oil is discharged from the relief opening parts, deteriorating the slidability of the relief valve.
The oil oozing out of the hole provided in the relief valve is discharged from the relief opening parts to the further deeper side and therefore cannot reach the back (deeper side) of the relief valve. Because the oil cannot readily reach the deeper side of the relief valve, the oil pressure on the deeper side of the relief valve decreases. Consequently, the spring located on the deeper side of the relief valve shrinks to a set value or more and slants over a long period of time, reducing its durability.